#if’ directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro. Its syntax is
#if expression controlled text #endif /* expression */
expression is a C expression of integer type, subject to stringent
restrictions. It may contain
||). The latter two obey the usual short-circuiting rules of standard C.
definedoperator, which lets you check whether macros are defined in the middle of an ‘
Identifiers that are not macros, which are all considered to be the
number zero. This allows you to write
#if MACRO instead of
#ifdef MACRO, if you know that MACRO, when defined, will
always have a nonzero value. Function-like macros used without their
function call parentheses are also treated as zero.
In some contexts this shortcut is undesirable. The
option causes GCC to warn whenever it encounters an identifier which is
not a macro in an ‘
The preprocessor does not know anything about types in the language.
sizeof operators are not recognized in ‘
enum constants. They will be taken as identifiers
which are not macros, and replaced by zero. In the case of
sizeof, this is likely to cause the expression to be invalid.
The preprocessor calculates the value of
expression. It carries
out all calculations in the widest integer type known to the compiler;
on most machines supported by GCC this is 64 bits. This is not the same
rule as the compiler uses to calculate the value of a constant
expression, and may give different results in some cases. If the value
comes out to be nonzero, the ‘
#if’ succeeds and the
controlled text is included; otherwise it is skipped.